Side Effects had lower domestic box office sales than The Sixth Sense because Bruce Willis is a well-known international actor from films like Pulp Fiction and Armageddon. People who saw him in the trailer for Side Effects may have been more likely to see the film assuming it would be good based on Willis' past roles in successful movies.

1. Side Effects has less
Domestic Box Office sales
than The sixth sense , I
believe this is because
Bruce Willis is a worldwide
actor known for his role in
Pulp fiction, Armageddon
and may others.
People would have seen
him in the trailer and if
they like him they would
then go to the cinema and
believe that because of
him they are seeing a good
film as they relate his
acting with quality films.
http://en.wikipedia.org/wiki/Bruce_Willis , http://www.the-numbers.com/movie/Side-
Effects-(2013)#tab=summary, http://www.the-numbers.com/movie/Sixth-Sense-
The#tab=summary
Sources:
Examples of Box office sales – Psychological Thriller Films: